The invention relates to a particle beam apparatus according to the preamble to claim 1.
In particular, the invention concerns a scanning electron microscope for low voltage applications. Low voltage microscopy is extremely important in the imaging of sensitive and non-conductive specimens. Because of the low energy (typically lower than 5 keV), resulting in low energy dissipation, sensitive specimens are not damaged. Insulating specimens can be imaged without distortion and disturbance, because insulators have a secondary electron yield of about 1 in the low energy range, which avoids or minimizes charging effects during the exposure with the particle beam. Low voltage microscopy, consequently, has great importance for the dimensional measurement and the inspection of device structures in the semiconductor manufacturing process.
Presently, high resolution low voltage microscopes are used for the above mentioned applications. High performance microscopes, as described in EP-B-0 333 018, use a combined electrostatic-magnetic immersion lens as final objective lens. By using the immersion principle, the primary beam path is at high energies. The final low beam energy is generated by deceleration in the objective lens, just in front of the specimen (EP-B-0 180 723). By applying such intermediate beam acceleration concepts, the electron-electron interaction inside the column, which broadens the beam and consequently decreases the resolution, can be significantly reduced.
The back-scattered and/or secondary electrons released by the primary electron at the specimen can be detected by a detector located in front of the objective lens. The arrangement of an in-lens or pre-lens detector has the advantage that the specimen can be located very close to the lens, resulting in a short working distance and correspondingly in a short focal length of the objective lens. A short focal length yields low chromatic and spherical aberration coefficients of the objective lens, which means high optical performance for the low voltage application.
The high performance low voltage arrangements according to the state of the art show a good optical performance, which even can even be improved by the application of objective lenses using a combined electrostatic retarding and magnetic single pole lens as described in U.S. Pat. No. 5,780,859. Those arrangements, however, have a drawback in secondary electron detection efficiency. Since the secondary electrons are accelerated by the retarding field for the primary particles, their energy is high and similar to the primary electron energy. Consequently, their behaviour is also similar to that of the primary electron beam. Accordingly, the secondary electron detection is difficult and not very efficient. Therefore, state of the art solutions either use coaxial detectors with small holes for the penetration of the primary beam (EP-B-0 333 018) or means for separation of the primary and the secondary electron beam (U.S. Pat. No. 5,422,486).
It is an object of the invention to provide a particle beam apparatus according to the preamble to claim 1 having an improved back-scattered and secondary electron detection efficiency.
This object is achieved according to the invention by the features of claim 1.
By applying the first and second additional means to decelerate and accelerate the primary particle beam in the region of the detecting means many kinds of effective detection systems can be used, because the back-scattered and/or secondary electrons are decelerated to their original energy distribution by applying a potential in the region of the detector which is close to the potential of the specimen.
Preferably the detector is located in a crossover of the primary been path. In this case, the optical effect of the first and second additional means on the primary beam performance is marginal and can be disregarded, because lenses or optical components in or near a crossover do not have any relevant effect on the beam characteristics.